Transformers are used in many types of electronic device to perform such functions as transforming impedances, linking single-ended circuitry with balanced circuitry or vice versa and providing electrical isolation. However, not all transformers have all of these properties. For example, an auto-transformer does not provide electrical isolation.
Transformers operating at audio and radio frequencies up to VHF are commonly built as coupled primary and secondary windings around a high permeability core. The core contains the magnetic flux and increases the coupling between the windings. A transformer operable in this frequency range can also be realized using an optical-coupler. An optical-coupler used in this mode is referred to in the art as an opto-isolator.
In transformers based on coupled windings or opto-couplers, the input electrical signal is converted to a different form (i.e., a magnetic flux or photons) that interacts with an appropriate transforming structure (i.e., another winding or a light detector), and is re-constituted as an electrical signal at the output. For example, an opto-coupler converts an input electrical signal to photons using a light-emitting diode. The photons pass through an optical fiber or free space that provides isolation. A photodiode illuminated by the photons generates an output electrical signal from the photon stream. The output electrical signal is a replica of the input electrical signal.
At UHF and microwave frequencies, coil-based transformers become impractical due to such factors as losses in the core, losses in the windings, capacitance between the windings, and a difficulty to make them small enough to prevent wavelength-related problems. Transformers for such frequencies are based on quarter-wavelength transmission lines, e.g., Marchand type, series input/parallel output connected lines, etc. Transformers also exist that are based on micro-machined coupled coils sets and are small enough that wavelength effects are unimportant. However such transformers have issues with high insertion loss.
All the transformers just described for use at UHF and microwave frequencies have dimensions that make them less desirable for use in modern miniature, high-density applications such as cellular telephones. Such transformers also tend to be high in cost because they are not capable of being manufactured by a batch process and because they are essentially an off-chip solution. Moreover, although such transformers typically have a bandwidth that is acceptable for use in cellular telephones, they typically have an insertion loss greater than 1 dB, which is too high.
Opto-couplers are not used at UHF and microwave frequencies due to the junction capacitance of the input LED, non-linearities inherent in the photodetector and insufficient isolation to give good common mode rejection.
What is needed, therefore, is a transformer capable of providing one or more of the following attributes at electrical frequencies in the range from UHF to microwave: impedance transformation, coupling between balanced and unbalanced circuits and electrical isolation. What is also needed is such a transformer that has a low insertion loss, a bandwidth sufficient to accommodate the frequency range of cellular telephone RF signals, for example, a size smaller than transformers currently used in cellular telephones and a low manufacturing cost.